Foil heating method



Filed May 24, 1966 FOIL HEATING METHOD ATTO INVENTOR LEONARD EDWARD FIELD May 6, 1969 Filed May 24, 1966 TEMP C9 6 G- o o L. E. FIELD 3,443,052

FOIL HEATING METHOD Sheet 2 012 s WATTS n.2, TI '1 25.5 WATTS /LB 4 l HOUR SOAKING PERIOD Ill]!!! 20 3O 4O 5O 6O 7O 80 TIME IN MINUTES INVENTOR LEONARD EDWARD FIELD United States Patent 3,443,052 FOIL HEATING METHOD Leonard Edward Field, Longfield, Kent, England, as-

signor to Aluminium Foils Limited, London, England, a corporation Continuation-impart of application Ser. No. 430,979, Feb. 8, 1965. This application May 24, 1966, Ser. No. 552,589

Int. Cl. H05b 3/00 US. Cl. 21950 11 Claims ABSTRACT OF THE DISCLOSURE A coil of thin metal strip having a natural thin adherent oxide coating is heated by an electrical current, a substantial proportion of which flows radially between radially spaced connections.

This application is a continuation-in-part of copending but now abandoned application Ser. No. 430,979 filed Feb. 8, 1965.

This invention is a method for heating coils of thin metal strip, especially aluminum foil. As used herein, aluminum includes alloys containing over fifty percent aluminum.

Aluminum foil often needs to be subjected to temperatures of a fairly high order for various reasons, such as evaporation of oil lubricants used in their fabrication, annealing of the metal, etc. Such toil is handled most conveniently in coils.

Heating of such coils has hitherto been effected in conventional annealing ovens to required temperatures, which may vary between 250 and 450 C. However, since aluminum foil is highly reflective, coils of such foil are slow to heat up when placed in a conventional annealing oven. Also, when the foil is to be stress relieved but not fully annealed, it is necessary for the temperature of the coil to be held below annealing temperatures, and this presents difficult problems of time and control, because if the oven is hot enough to heat the coil quickly it may overheat it, and if the oven is at exactly the stress relieving temperature it will take a long time to bring the coil to the same temperature.

In accordance with the present invention, these difficulties are overcome by using the electrical resistance characteristics of the natural oxide film on aluminum foil to provide enough resistance radially across the coil to allow it to heat up when a relatively small voltage is impressed across the inner and outer peripheries of the coil. Such heating is not substantially dissipated by radiation from the coil, due to the low emissivity of its bright surfaces, and the heating operation can therefore be performed in an open room at 70 F. ambient temperature, for example. Heating can be rapid, and once the desired temperature is reached further heating can be stopped, or can be reduced sufficiently to maintain a desired temperature, by stopping or reducing the current fiow through the coil.

The method of this invention is applicable to aluminum foils having thin adherent oxide layers and varying in thickness from about 0.0002 to about 0.004 inch. Thicker foils require too much current fiow for practical purposes, while thinner foils are likely to give a greater temperature difference from the core to the outside.

The method is particularly applicable to foils consisting of 99 percent or more aluminum or to aluminum alloys in which aluminum is the primary constituent, for example, the 1.25 percent manganese alloy known as N53 or US. 3003 alloy. The foil to be treated generally is in the normal condition as rolled, and includes any normal oxide coating. The foil may be matt on one side and shiny Patented May 6, 1969 on the other and'when coiled in the double-layered state may be treated as such or after separation of the layers. The coil often will include a residue of rolling lubricant, but need have no oil residue for treatment in accordance with the invention.

The preferred electrical potential is from about 3 to about 50 volts. It will usually not be found necessary to use a voltage above about 40 volts, and voltages above 50 volts are generally unnecessary and undesirable. Enough wattage, depending upon the weight of the coil, is employed to attain the desired temperature. While alternating current is preferred, in some cases DC. current has been used, with very little difference in effect being observed. Experiments have shown that in most cases a power factor of 0.9 can be achieved.

For purposes of the invention the foil is wound with the natural oxide coating on one face in contact with the natural oxide coating on the other face, with no intermediate layer of insulation (the residual oil being considered insignificant in this regard). It has been found that the electrical current is conducted between the inner and outer peripheries of the coil both spirally along the length of the foil strip, and radially by passage of the current directly between adjacent convolutions of the coil through the oxide film. Thus the current may be considered as passing through parallel paths, one being spirally through the coiled strip, which has a resistance being a function of the cross-sectional area and length of this conductive path and of the conductivity of the metal, and the other path being radially through the coil, whichhas a resistance which is a function of the relatively highly resistive oxide interfaces and of the relatively short length of the radial path.

The major proportion of the current has been found to take the radial path through the coil. The total resistance of the coil and the exact distribution of current between the radial and spiral path depends on a number of factors such as the thickness of the foil, the pressure between adjacent windings of the reel, the thickness of the oxide layer on the foil and the relationship of size between the inner and outer diameters of the coil. While 50 percent or more of the total current will pass through the radial path, it has been found that usually less than 5 percent of the current will take the spiral path through the sheet material. In experiments reported below in Examples VVII it was found that only about 0.25 to 2 percent of the current passes through the spiral path when using aluminum foils of common gauge.

As regards the effect of the oxide film on the resistance of the reel, the normal tightly adherent layer of natural oxide on aluminum foil after the rolling operation produces the desired results. Variation in resistance within the reel of sheet material may be apparent, however, due to variation in pressure between adjacent windings, which variation is greater with greater width. Uneven heating within the coil can be compensated to a certain extent by using a slower rate of heating or intermittent heating so that heat conduction within the coil maintains an even temperature. It appears desirable to keep the coil temperature somewhat below about 425 C. during the operation of the invention, since there is some indication that the electrical characteristics of the oxide film may be changing above this temperature under some conditions, perhaps due to decreases in the oxide film resistance.

Even if there is no difference in winding tightness across the web in a given spool, there may be variations between one spool and another, so that the rate of heating of two spools otherwise identical may vary. Usually, however, this is not so important, as maintenance of a given final temperature is required rather than a completely comparable rate of heating.

Some variation of the overall resistance of a reel of foil occurs during heating, with corresponding variation in the wattage utilized. This may be due to expansion of the reel and corresponding change in interturn pressure or to the evaporation of oil from between the windings or change in oxide thickness or to any combination of these effects.

It is apparent, of course, that a suitable resistance exists in each path if the desired heating is to be obtained. A greater length of sheet material will offer greater resistance in the foil path, but also will usually provide more coils for increased resistance in the more direct flow path.

The invention will be better understood by reference, for purposes of illustration only, to the accompanying drawings, in which FIGURE 1 shows apparatus which may be used in carrying out the process of the invention and in which FIGURE 2 is a graph showing the temperature of a coil plotted against time in a typical heating run according to the invention.

In FIGURE 1, coil of aluminum foil is wound on and is in good electrical contact with the central portion of hollow steel core 12. The ends of core 12 protrude from coil 10 and rest upon the cradles 14 of steel stillage 16, which in turn rests upon brass angle collectors 18. Directly above coil 10 is stationary pneumatic cylinder fixed to channel 22, which is in turn fixed to wall 24. Piston 26 fitting within cylinder 20 is attached by rod 28 to heavy copper connecting bar 30 which has been plated to prevent oxidation. By adjusting the admission and release of air through lines 32 and 34, connecting bar 30 is forced downwardly. Connecting bar 30 compresses a pad 36 of soft annealed aluminium foil into good electrical contact with the top surface of coil 10, and urges core 12 into good electrical contact with cradles 14. Connecting bar 30 and pad 36 are electrically connected by conductors 38 to bus bars 40 and 42. Alternating current is supplied from any suitable source to angle collectors 18 and bus bars 40 and 42 by adjustable auto transformer 44, main transformer 46, and lines 48 and 50. In the interest of safety the secondary of transformer 46 is earthed as shown in FIG. 1. Rod 28 is insulated from 30. Thermocouples may be connected to coil 10 to check on the temperatures at various points therein; for example, bow-type thermocouple 52 may be attached to the surface of the coil. In addition, the thermocouples may be associated with the adjustable auto transformer for automatic control of heating.

Core 12 is used for lifting coil 10, and preferably has an outer diameter at least as large as the radial thickness of the coil. This reduces the diiference between current density of radially flowing current adjacent the outer periphery of the coil as compared to that adjacent the inner periphery, and thus reduces the tendency to heat a little faster progressively toward the inner periphery of the coil.

A number of coils may be placed on a common rod, their external surfaces being linked so that they are connected in parallel. It would in such case be necessary for all the coils to be similar in resistance characteristics to give a roughly similar current flow in each coil.

The following examples illustrate the invention.

Example 1 In this example a coil of aluminum foil 0.02 mm. in thickness was annealed to soft temper by the process of this invention and so as to leave a slick of lubricant on the foil.

The dimensions of the coil were: overall diameter, 10 /2 inches; width, 12 inches; weight (excluding core), 86 pounds. The coil was wound on a hollow steel core of internal diameter 3 inches and 0.048 inch in thickness.

A bright steel rod was inserted in this hollow core, to which rod one electric terminal was attached, and the outer connection was made by a counterweighted copper bar terminal resting on the outer convolution of the foil. The two terminals were connected to a source of direct current for two hours, during which time the average applied voltage was 6.3 volts and the average applied current was 350 amps. The temperature of the foil at the end of this time was 300 C. The current was then switched off and the coil allowed to cool to room temperature in air.

The average power supplied to each pound of foil was 25.75 watts.

Example II A coil of aluminum foil 0.01 mm. in thickness was heated to soft temper and to the oil-free state by the following process. The dimensions of the coil were: overall diameter, 7% inches; width, 19 inches; weight (excluding core), 74 pounds; internal diameter of steel core, 2% inches; thickness of core, 0.048 inch.

The coil was connected for four hours to a source of electricity in the manner described in Example I, by which time the temperature of the foil reached 355 C. The foil was maintained at this temperature for one hour by switching the current on and off, after which it was allowed to cool to room temperature in air.

The average applied voltage during the process was 5.3 volts, and the average applied current 365 amps.

The average power per pound of foil was 26.4 watts.

Example III A reel of aluminum foil (US 3003 alloy) 0.08 mm. in thickness was heated with the object of holding the temperature precisely at 255 C. to produce H26 temper. This alloy has improved elongation in the sheet, making it suitable for the manufacture of stamped products such as pie plates. The coil had an overall diameter of 18 inches, a width of 21 inches and a weight, excluding the conductive steel core of about 450 pounds.

The coil was connected to a source of electricity as described in Example I, and the heating process was carried out in two stages. First, the coil was subjected to a high wattage for 60 minutes, in order to raise its temperature to 255 C. and then the voltage was reduced to that just sufficient to compensate for heat losses and gtisej ctil was given a soaking period of one hour at Stage I Average Volts 28.25 Amps 414 Watts per pound of foil 25.5

Stage 11 Volts 15 Amps 180 Watts per pound of foil 6 The heating cycle of Example III is illustrated graphically 1n FIGURE 2 of the drawings. The temperature of the metal was measured at points of the coil corresponding to the placement of thermocouples 54, 56, 58, and 60 of FIGURE 1. The curves of FIGURE 2 show the temperature gradients for each of the points indicated. As will be seen, the point 54 has a temperature lead for almost the whole time of Stage 1 of the heating and converges with 56, 58, and 60 at approximately 255 C. This lead in temperature was checked just before the end of stage 1 (at the point marked by reducing the wattage sufficiently to allow the coil temperature to even out by conduction but at the same time retaining the wattage sufiiciently hlgh to maintain the temperature at the required level.

Example IV A coil of aluminum foil 0.009 mm. in thickness was heated to a temperature of 300 'C. The dimensions of the reel were: overall diameter, 6 inches; width, 7% inches; weight (excluding core), 14 pounds.

The coil was connected for one and three quarter hours to a source of electricity in the manner described in Example I. The average applied voltage was 28.2 volts and the average current was 24.5 amps. Average watts per pound of foil 48.7.

Example V Aluminum foil 0.008 millimeter thick and 18 inches wide was wound into a coil, the total weight of which was 110 pounds, outside diameter 10.375 inches, inside diameter 4.5 inches. The electrical resistance of this coil was tested and found to be 0.105 ohm at room temperature and 0.123 ohm at 135 C. At 16 volts, the current passing through the coil is 130 amperes.

This coil was unwound and rewound but interleaved with a polyethylene insulating sheet. The resulting rewound sheet was measured for electrical resistance and found to be 33 ohms. The calculated current in the spiral path at 16 volts was =0.485 amp. It is calculated from these figures that only 0.374 percent of the current passed spirally through the original noninterleaved coil, the remainder of current passing radially between the inside and the outside of coil.

Example VI Aluminum foil 0.0 25 mm. thick by 20 inches wide was wound around a steel core 4 /2 inches in diameter. The net weight of the foil was 150 pounds and the coil had an outside diameter of about 11 /2 inches and an electrical resistance of about 0.02 ohm, allowing passage of 360 amps current at 8.5 volts. When unwound and rewound interleaved with polyethylene film as in Example V, the resistance was measured at 5.14 ohms, allowing passage in the spiral path of only 1.653 amps at 8.5 volts. In the interleaved coil, therefore, only about 0.46 percent of the total current passes through the foil path, the rest traveling radially.

Example VII 267 pounds of aluminum foil 0.076 mm. thick and 23 /8 inches wide were wound around a steel core 4 /2 inches in diameter. The outside diameter of this coil was 13 inches and its resistance measured about 0.01 ohm. At 5.2 volts about 500 amperes current passes through the coil.

When interleaved with polyethylene and rewound as in Example V, the resistance was found to be 0.625 ohm. This resistance would permit passage of only about 8.32 amps current at 5.2 volts and thus only 1.665 percent of the current in the non-interleaved coil was found to pass in the spiral path.

Example VIII A strip of 3003 (aluminum magnesium) alloy foil .08 mm. thick, 24 inches Wide, and weighing 1300 pounds was wound into a 16 inch diameter core and heated to produce an H216 temper as shown in the following table:

Voltage Current Temperature Time Watts in in (in in per volts amps 0.) minutes pound 1 Soaking.

As shown, a temperature of 260 C. was attained and held for four hours, the temperature being held at 100 C., 150 C. and 200 C. for short periods to allow heat to flow through the coil by normal conduction. This is not usually necessary for full annealing as distinct from tempering. Tensile tests were made on the metal and the results indicated that this particular coil was slightly on the soft side. Following coils were heated at a lower temperature of 255 C. and gave better results. This example indicates the importance of close temperature control for temper let-down treatments and the coil heating method has the distinct advantage of controlling the metal to the close limits required for this treatment. The heat is generated within the coil and the metal being hotter than the surrounding air, there is no increase in its temperature after the control has cut out. As a very large proportion of current flow is in a radial direction, it is beneficial to have a closer outside diameter/inside diameter ratio in order to prevent current saturation towards the core. Use of the large diameter core has the added advantage of being able to wind a larger weight of foil on the core for a smaller build-up allowing a more uniform heating by natural conduction.

Example IX In order to be fully annealed and to have the rolling lubricant removed, a strip of commercial purity aluminum .05 mm. thick, 22% inches wide, and weighing 786 pounds was wound onto a 9 inch diameter core and heated by the apparatus shown in FIGURE 1 as shown in the following table, the temperatures being those recorded by bow-type thermocouple 52 attached to the outer surface of the coil:

Voltage Current in Temperature Time in Watts per in volts amps (in C.) minutes pound (Continued controlling temperature at 375 C. for another 9 hours and 40 minutes. As is shown, the coil was heated to a temperature of 375 C. and held at that temperature for 15 hours. This was done to completely free the surface of oil.)

As will be seen from the above examples, the method according to the invention is applicable to coils which vary greatly in dimensions. It should be noted, however, that the heating efiiciency is lower in coils of small dimensions, see Example IV. In order to apply the method to any given coil, it is convenient first to calculate or read from a graph the power in watts required to be utilized according to the weight of the reel in order to give the required rate of heating. The voltage to be applied to the coil can then be determined, if necessary. 20-25 watts per pound is a usual power requirement for obtaining the desired temperature, but lower watts per pound may also be used; as for example, in Examples VIII and IX above. The power requirements for maintaining a temperature are usually much lower.

A flexible voltage supply should be available and at the commencement a voltage should be applied which is below that actually required, the voltage being increased as the process proceeds. In order to obtain maximum required voltage for minimum current, the higher voltages within the range specified above are preferred as stated.

As shown, the method according to this invention has the advantage that, at least with heavy gauge sheet, i.e., 0.050.1 mm. in thickness, uniform heating can be achieved through the coil and that the temperature of the foil can be easily and accurately controlled. The method can also be used with gauges down to 0.009 mm, Also the method may be carried out in safety, since high voltages are not necessary. The heat-treating process of this invention is economical in electricity, for only the metal foil is heated and not the mass of an oven. The spool can be open to the air so that oil vapours can escape. Alternatively, the method may be adapted for vacuum annealing, a process which has hitherto been very difiicult to carry out, especially with aluminum in a bright condition, which is difiicult to heat by radiation. Finally the equipment needed is low in capital cost as compared with annealing ovens.

While present preferred embodiments of the invention have been illustrated and described, it will be understood that the invention may be otherwise variously embodied and practiced within the scope of the following claims.

What is claimed is:

1. A method for heating a coil of thin metal strip having a natural thin adherent oxide coating on the opposite faces of the strip, said strip being wound with said oxide coating of one face in contact with the said oxide coating of the other face, and said strip having a length and cross sectional area sufficient to provide a substantial electrical resistance helically through the length of the metal strip relative to the electrical resistance radially through the coil, comprising the steps of connecting a first portion of the coil to one side of an electric power circuit and connecting a second portion of the coil to the other side of the circuit, said second portion being radially spaced from said first portion, and establishing a voltage drop across, and consequent flow of substantial current radially "between, said first and second portions of the coil, whereby the coil is heated and its temperature is raised.

2. The method of claim 1 wherein metal strip is aluminum foil.

3. The method of claim 1 in which the current is alternating current.

4. The method of claim 1 in which the current is direct current.

5. A method for heating a coil of aluminum foil about 0.0007 to 0.004 inch thick having a natural thin adherent oxide coating on the opposite faces of the foil, said foil being wound with said oxide coating of one face in contact with the said oxide coating of the other face, and said foil having a length and cross sectional area suflicient to provide a substantial electrical resistance helically through the length of the foil relative to the electrical resistance radially through the coil, comprising the steps of connecting a first portion of the coil to one side of an electric power circuit and connecting a second portion of the coil to the other side of the circuit, said second portion being radially spaced from said first portion, and establishing a voltage drop across, and consequent flow of current primarily radially between, said first and second portions of the coil, whereby the coil is heated and its temperature is raised.

6. The method of claim 5 wherein said first portion is the outer periphery of the coil and said second portion is the inner periphery of the coil.

7. The method of claim 5 in which the voltage drop is about 3-50 volts.

8. The method of claim 5 in which the inner diameter of the coil is larger than the radial thickness of the coil.

9. The method of claim 5 in which the coil is in an atmosphere at room temperature while it is being heated.

10. The method of claim 5 in which the coils temperature is raised high enough to stress relieve the coil without annealing it.

11. The method of claim 5 in which the temperature of the coil does not rise above about 425 C.

References Cited UNITED STATES PATENTS 2,308,995 1/1943 Miess 2195O 2,959,663 11/1960 Fenn 21927O 2,975,262 3/1961 Schnick 219270 RICHARD M. WOOD Primary Examiner.

B. A. STEIN, Assistant Examiner. 

